Aluminum electrode electrolytic capacitor construction

ABSTRACT

BOTH THE PASSIVATION OF ALUMINUM FOIL PARTICULARLY FOR USE AS ELECTRODE IN WET ELECTROLYTIC CAPACITORS AND THE INHIBITION OF THE ALUMINUM-WATER-REACTION WITHIN THE COMPLETED CAPACITOR ARE DESCRIBED. PRETREATMENT OF ETCHED ALUMINUM FOIL INCLUDED THE COMBINED STEPS OF IMMERSION IN A BOILING AQUEOUS SOLUTION CONTAINING CRO3 AND H3PO4 FOLLOWED BY A 3-MINUTE IMMERSION IN A BOILING SOLUTION OF CONTROLLED PH CONTAINING H2PO4-, SIO2= AND CRO3,   INHIBITION OF ALUMINUM IN A WET ELECTROLTIC CAPACITOR IS AFFORDED BY THE ADDITION OF A CONCENTRATION OF AT LEAST ONE OF THE FOLLOWING INHIBITING IONS TO THE LIQUID ELECTROLYTEL ; PHOSPHATE, PHOSPHITE, PERIODATE, ARSENATE, TUNGSTATE, SILICATE VANADATE (VANDADATE), TELLURATE, TELLURITE, ANTIMONATE, ARSENITE, SELENATE, SULFITE AND GERMANATE.

May 21, 1974 o. A. VERMILYEA Er AL Re. 28,015

ALUMI NUM ELECTRODE ELECTROLYT I C CAPACITOR GONSTRUCT ION F/GI OriginalFiled Dec. 3, 1969 FIG. 2

ALUMINUM FOIL CLEANED A/VD ETC'HED FOIL IMMERSED //v BOILING 61 0 /11P04 501. U T ION FG/L IMMERSED l/V BOILING SOLl/T/O/V OF H POLS/O ,603

ELECTRODE ASSEMBLY PREPARED ASSEMBL V IMPREGWA TED WITH F/LL ELECTROLYTECO/V T A l/V/IVG 0X YA N/O/V SOURCE IMPRE'GNA r50 ASSEMBL r AGED ASSEMBLY CANNED A/VD SEALED IN VE N TORS" 04 W0 4. VERM/L YEA 00mm R. OCHAI?W/L L EM VEDDER bra-4 THE/R ATTORNEY United States Patent 28,015ALUMINUM ELECTRODE ELECTROLYTIC CAPACITOR CONSTRUCTION David A.Vermilyea, Schenectady, N.Y., Donald R. Ochar, Columbia, S.C., andWillem Vedder, Albany, N.Y., by General Electric Co., New York, N.Y.Original No. 3,622,843, dated Nov. 23, 1971, Ser. No. 881,853, Dec. 3,1969. Application for reissue Sept. 13, 1972, Ser. N 0. 288,508

Int. Cl. H01g 13/00 US. Cl. 29-570 6 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE Both the passivation of aluminum foilparticularly for use as electrodes in wet electrolytic capacitors andthe inhibition of the aluminum-water reaction Within the completedcapacitor are described. Pretreatment of etched aluminum foil includesthe combined steps of immersion in a boiling aqueous solution containingCrO and H PO, followed by a 3-minute immersion in a boiling solution ofcontrolled pH containing H POf, SiO and CrO Inhibition of aluminum in awet electrolytic capacitor is afforded by the addition of aconcentration of at least one of the following inhibiting ions to theliquid electrolyte: phosphate, phosphite, periodate, arsenate,tungstate, silicate vanadate [vandadate], tellurate, tellurite,antimonate, arsenite, selenate, sulfite and germanate.

BACKGROUND OF THE INVENTION The capacity of an electrolytic capacitor isdetermined primarily by the area of the electrode (usually the anode)surface and the thickness of the dielectric film covering this surface.The typical wet electrolytic capacitor employs an etched surfaceelectrode structure and a number of methods have been developed toproduce the desired roughening of the electrode surface. The mosteffective of these methods is the electrolytic etching of the electrodefoil, while the foil is immersed in an etching solution e.g. sodiumchloride (followed in some cases, e.g. for highvoltage applications byetching in nitric acid). In the process of electrolytic etching the foilis made the positive electrode with respect to a second electrodeimmersed in the same etching solution. The resulting attack upon thealuminum develops pores extending into the foil, these pores rangingfrom about 1-5 microns in lateral dimension and penetrating to a depthof from about 100- 500 microns.

It has been found that aluminum foil etched for usage as electrodes inelectrolytic capacitors loses capacitance during storage. This loss incapacitance is believed to be the result of blockage of some or all ofthe small pores by the formation therein of aluminum hydroxide, acorrosion product of the aluminum/water reaction. Water for thisdestructive reaction is most often present in the atmosphere of thestorage facilities and in event of temperature cycling liquid water maycondense within these small pores. Aluminum oxidation in liquid waterproduces a layer of amorphous oxide, which dissolves and reprecipitatesas a porous layer of hydroxide of (AlOOH), which may grow to a thicknessof several microns depending upon the temperature. Development of thiscorrosion product within the small pores, therefore, decreases theavailable electrode area. Prevention of the aluminum hydroxide formationwould, therefore, be beneficial to minimizing reduction in capacitanceduring storage.

ice

After etching, the electrodes are cleaned, usually by rinsing in water,to remove all traces of any contaminating or foreign materials, whichmight either affect forrnation of the requisite dielectric film oraffect the performance of the finished capacitor. After cleaning, thealuminum foil to be used as the anode is anodized to form an anodicoxide (predominately amorphous A1 0 layer thereon using either a basicor an acidic anodizing electrolyte. Numerous electrolytes for theformation of anodic films are listed on page 55 of the test bookElectrolytic Capacitors by Paul McKnight Deeley (Recorder Press 1938).The most commonly used anodizing electrolytes are boric acid andinorganic phosphates.

The anodic oxide layer formed substantially uniformly covers all of theetched surface (both sides) of the aluminum foil even extending into andcovering the inside surfaces of the small pores.

When the active dielectric film of aluminum oxide has been formed, atypical electrode assembly (anode/paper spacer/cathode/paper spacer) isprepared. This assembly is rolled into a cylindrical configuration andimpregnated with (as by immersion) a fill electrolyte for aging." Duringthe aging process a potential is applied to the electrode assembly equalto or slightly in excess of the rated operating voltage for thecompleted capacitor for a predetermined period of time. The aging, orreforming, potential tends to repair any breakage or cracking, which mayhave occurred in the anodic film during the handling and assembly.

When the aging has been completed, the electrode assembly is insertedinto an aluminum can and the can is capped and sealed.

Fill electrolytes (as distinguished from anodizing lectrolytes) areeither nonaqueous (organic solvent) or have a low-water content.Ethylene glycol is frequently used, because of its solventcharacteristics and low freezing point.

During sustained periods, when the completed capacitor is stored or isstanding on open circuit, the anodic oxide layer is constantly subjectto attack by water deliberately or spuriously present in the fillelectrolyte. This degradation according to the aluminum/water reactioncan reduce the dielectric strength of the capacitor to such a level thatthe imposition of a normal electrical load may cause failure of thecapacitor.

It is to this problem of open circuit dielectric degradation as well asto the problem of degradation of etched aluminum foil during storagethat the instant invention is directed for the purpose of optimizingelectrolytic capacitor construction.

In describing this invention the terms passivation and inhibition" areemployed. These terms are defined as follows:

Passivation is the rendering of a metallic or metallic oxide surfacerelatively immune to corrosive attack in a normally hostile environment.

Inhibition is the rendering innocuous of a normally hostile environmentby the addition of substances to the environment.

SUMMARY OF THE INVENTION The instant invention introduces into thesequence of steps in preparing a wet electrolytic capacitor (a) amodified pretreatment of the etched aluminum foil to passivate theetched surface in order to delay the initiation of hydroxide formationfrom reaction of the aluminum with water and/or (b) the addition of oneor more substances of a specific class, which provide inhibitor ions, tothe fill electrolyte such that the completed electrolytic capacitor isafforded protection against dissolution of the dielectric oxide duringperiods of electrical inactivity.

3 BRIEF DESCRIPTION OF THE DRAWING The exact nature of this invention asWell as objects and advantages thereof will be readily apparent fromconsideration of the following specification relating to the annexeddrawing in which:

FIG. 1 is an exploded view of an example of an electrolytic capacitor towhich the instant invention applies and FIG. 2 is a diagrammaticrepresentation of the process of preparation of electrolytic capacitorsaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As is shown in FIG. 1, capacitor10 comprises cooperating electrodes 11, 12 in the form of thin aluminumfoils. These electrodes have a thin film of aluminum oxide thereon.Between electrodes 11 and 12 as they are arranged in the electrodeassembly are disposed spacers 13 consisting of sheets of absorbentmaterial. Spacers 13 are, or become, impregnated with electrolyticcomposition. Terminal tabs 14, 16 are provided for establishingelectrical contact to the electrodes. After preparation, the electrodeassembly is placed in aluminum can 17, which is sealed with cap 18provided with external terminals 14a, 16a (connected to the tabs 14, 16,respectively) and vent plug hole 19 for the release of excess internalpressure.

As is described in FIG. 2, after etching and cleaning and prior tostorage or anodizing, the aluminum foil is subjected to a sequence oftwo steps, which produces substantial passivation of the aluminum foil.

In the first step of the sequence the etched aluminum foil is immersedin a boiling aqueous solution containing Cro and H PO This immersionreduces the thickness of the thin A1 layer (normally present on thesurface of aluminum) after which a complex region develops at thesurface of the A1 0 layer as this layer reaches an equilibriumcondition, when the rate of layer buildup equals the rate of layerdissolution. The complex region at the surface of the A1 0 layercontains CrO and P0 but is incapable of rendering the system highlypassive. It is essential that this stripping operation be accomplishedfirst and be followed by the second step.

Once equilibrium has been reached the second step may be conducted. Inthis step the stripped etched aluminum foil is immersed in a boilingaqueous solution containing CrO H PO and SiO (with pH adjusted to avalue ranging from 4 to 6) which exposure changes the complex layer atleast by including SiO as a component thereof.

In a typical passivation sequence a conventional stripping solution (20grams CrO and 35 milliliters of concentrated H PO per liter of water)may be used with the immersion lasting from 1 to 5 minutes followed by a3 minute immersion in an aqueous solution of adjusted pH containing fromabout to 10- moles per liter of each of NaH PO Na SiO and CrO Use ofthis pretreatment sequence has shown that initiation of hydroxideformation according to the aluminum/water reaction is delayed by atleast a factor of 10 as compared to untreated foil and by at least afactor of 10 compared to aluminum foil subjected to the first, but notthe second, step of this sequence.

The above-described procedure for passivating aluminum foil may beapplied to either etched or plain foil. however, the prime object hasbeen to prevent destruction during storage of the high-surface areadeveloped by the etching process.

The passivated aluminum foil may be stored at considerably reduced riskuntil ready for the anodizing step preparatory to constructing theelectrolytic capacitor.

In addition, when the internal construction of the electrolyticcapacitor 10 has been prepared, rolled, aged and inserted into thealuminum can 17, the corrosion process (which will otherwise occurwithin the sealed capacitor between the exposed aluminum surfaces andliquid water in the electrolyte and between the anodic oxide layer andliquid Water) may be inhibited by introducing certain inorganicsubstances into the capacitor can dissolved in the fill electrolyte.

Thus, it has been determined that inhibition will be strong when (a) inthe oxyanion of the inhibitor the centrally located, positively chargedcomponent (for example, phosphorous in the H PO ion) has a radiusbetween 0.24 and 0.5 times the radius of oxygen and when (b) theinhibiting oxyanion has a proton level of 0.4 to 0.7 ev. relative tothat of a proton in H 0 In essence, the structure and proton level ofthe inhibitor ion should be close to the structure and proton level ofaluminum oxide. The substances which have the required structure andproton level are tellurate, tellurite, tungstate, antimonate, vanadate,arsenate, arsenire, [aresenate, aresenite] selenate, sulfite, periodate,phosphate, phosphite, germanate and silicate ions. All of thesesubstances are effective inhibitors, the strongest being phosphate,phosphite, periodate, arsenate, tungstate, silicate and vanadate in theorder given. The pH of maximum effectiveness is about 5:1. The oxyanionconcentration dissolved in the fill electrolyte should be in the rangeof from about l() to 10 weight percent of the electrolyte solution withthe preferred range being from about 0.1 to about 1.0 weight percent.

Thus, by adding to the fill electrolyte soluble salts, which arecompatible with the fill electrolyte and which provide concentrations ofions of one or more of the above materials (eg the sodium, potassium,calcium, ammonium salts thereof) so that the concentration of requisiteions remains within the sealed electrolytic capacitor, initiation of thecorrosion process is delayed by at least a factor of compared toelectrolytic capacitors, which do not contain the aforementionedinhibiting ions.

Examples of fill electrolytes are as follows (percentages are byweight).

A. Organic solvent electrolyte (with organic and inorganic salts)Ethylene glycol 1 49.8 Boric acid 10.0 Pyrogallol 10.0 Methylamine 30.0Sodium phosphate 0.2

B. Organic solvent electrolyte (with organic salts) Dimethylformamide98.7 Oxalic acid 1.0 Ammonium oxalate 0.1 Ammonium phosphate 0.2

C. Organic solvent electrolyte (with inorganic salts) Ethylene glycol89.8 Ammonium biborate 10.0 Ammonium phosphite 0.2

D. Aqueous electrolyte (organic salt+water) Ammonium tartrate 10.0 Water89.8 Ammonium periodate 0.2

What we claim as new and desire to secure by Letters Patent of theUnited States:

1. In the preparation of a wet electrolytic capacitor wherein thefollowing steps are performed: etching aluminum foil; cleaning saidetched foil; anodizing said etched, cleaned foil; preparing an electrodeassembly wherein anode and cathode layers are separated by spacers, saidanode layer being made of said anodized foil; irnpregnating saidelectrode assembly with a fill electrolyte; aging said electrodeassembly by the application of electrical potential thereto; introducingsaid "aged electrode assembly into a cam; and capping and sealing saidcan, the improvement in said series of steps comprising:

a. immersing the cleaned etched foil in a first boiling aqueous solutioncontaining Cr0 and H PO b. immersing said foil so modified in a secondboiling aqueous solution containing from about to 10- moles per liter ofeach of HgPOf', Si0 and CrO and c. dissolving in the fill electrolyte awater-soluble source of inhibiting oxyanions, said oxyanions beingpresent in a concentration in the range of from about 10- to about 10percent by weight of the electrolyte solution and being selected fromthe group consisting of tellurate, tellurite, tungstate, antimonate,vanadate, arsenate, arsenite, selenate, sulfite, periodate, phosphate,phosphite, germanate and silicate ions.

2. The improvement of claim 1 wherein the second boiling solutioncontains NaH PO Na SiO and CrO 3. The improvement of claim 1 wherein thepH of the boiling solution is adjusted in the range of from 4 to 6.

[4. In the preparation of a wet electrolytic capacitor wherein thefollowing steps are performed: etching aluminum foil; cleaning saidetched foil; anodizing said etched, cleaned foil; preparing an electrodeassembly wherein anode and cathode layers are separated by spacers, saidanode layer being made of said anodized foil; impregnating saidelectrode assembly with a fill electrolyte; aging said electrodeassembly by the application of electrical potential thereto; introducingsaid aged electrode assembly into a can; and capping and sealing saidcan, the improvement in said series of steps comprising:

a. dissolving in the fill electrolyte a water-soluble source ofinhibiting oxyanions, said oxyanions being present in a concentration inthe range of from about l0- to about 10 percent by weight of theelectrolyte solution and being selected from the group consisting oftellurate, tellurite, tungstate, antimonate, arsenate, arsenite,selenate, sulfite, periodate, and germanate ions] [5. The improvement ofclaim 4 wherein the pH of the fill electrolyte is in the range of from 4to 6.]

[6. The improvement of claim 4 wherein the concentration of dissolvedoxyanions is in the range of from about 0.1 to about 1.0 percent byweight of the electrolyte solution] 7. In the preparation of a wetelectrolytic capacitor wherein the following steps are performed:etching aluminum foil; cleaning said etched foil; anodizing said etched,cleaned foil; preparing an electrode assembly wherein anode and cathodelayers are separated by spacers, said anode layer being made of saidanodized foil; impregnating said electrode assembly with a fillelectrolyte; aging said electrode assembly by the application ofelectrical potential thereto; introducing said "aged electrode assemblyinto a can; and capping and sealing said can, the improvement in saidseries of steps comprising:

a. immersing the cleaned etched foil in a first boiling aqueous solutioncontaining CrO, and H -PO and b. immersing the foil as modified by step(a) in a second boiling aqueous solution containing from about 10* to10*" moles per liter of each of H POF, SiO and CrO;

whereby etched aluminum foil may be stored without substantialdeterioration prior to the anodizing step.

8. The improvement of claim 7 wherein the second boiling solutioncontains NaH PO Na SiO and CrO 9. A method for the passivation of analuminum surface comprising the steps of:

a. immersing the aluminum surface in a first boiling aqueous solutioncontaining CrO and H PO and b. immersing the aluminum surface asmodified by step (a) in a second boiling aqueous solution containingfrom about 10- to 10* moles per liter of each of H PO [and] SiO and CrO[10. In a wet electrolytic capacitor wherein a housing contains ananodized aluminum anode and a liquid electrolyte and means are attachedto said housing for the forming of an external electrical connectionthereto, said housing being sealed to prevent leakage of the electrolytetherefrom, the improvement in said combination wherein a water-solublesource of inhibitive ions is disposed in said housing with saidelectrolyte, said inhibitive ions being present in said electrolyte in aconcentration in the range of from about 10* to about 10 percent byweight of electrolyte solution and being selected from the groupconsisting of tellurate, tellurite, tungstate, antimonate, arsenate,arsenite, selenate, sulfite, periodate, and germanate ions] 11. Theimprovement of claim 6 wherein the pH of the fill electrolyte is in therange of from 4 to 6.]

[12. The improvement of claim 10 wherein the concentration of dissolvedions is in the range of from about 0.1 to about 1.0 percent by weight ofthe electrolyte solution.]

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,329,873 7/1967 Hagihara et a1 317-230 2,890,3946/1959 Stephenson et al. 317-230 2,875,384 2/1959 Wallmark 317-2343,345,544 10/1967 Metcalfe 317-230 3,502,947 3/1970 Hand 317-2302,965,816 12/1960 Ross 317-230 3,547,423 12/1970 Jenny et al 317-2302,019,154 4/1936 Emmens et a1 317-230 2,757,140 7/1956 Bush 317-230 X2,934,682 4/1960 Schwartz et al. 317-230 2,994,809 8/1961 Jenny et al.317-230 3,003,089 10/1961 Bernard et al. 317-230 3,138,746 6/1964 Burgeret al. 317-230 2,934,681 4/1960 Ross 317-230 OTHER REFERENCES PeriodicTable of the Elements, E. H. Sargent & Co., Chicago, 1964.

RUDOLPH V. ROLINEC, Primary Examiner W. D. LARKINS, Assistant ExaminerUS. Cl. X.R.

